The circuit of the present invention is an improvement on the circuit shown in U.S. Pat. No. 3,916,222 which issued to James B. Compton on Oct. 28, 1975. The basic element is a field effect transistor (FET) that is switched by means of its gate potential so that it will be either in a high resistance off state or a low resistance on state. In effect the FET is a SPST switch that is toggled by means of a control current. Such devices are very useful in switching analog signals because the circuits can be arranged so that the FET gate voltage or control can be made to track the source voltage, thus ensuring positive control over an analog voltage that can have a component of varying magnitude. The control circuits can be designed to switch the FET very rapidly and to dissipate relatively little steady state power. Finally the entire switch including control circuits can be fabricated into a common semiconductor substrate using conventional I.C. techniques. In fact it is common to fabricate a plurality of switch circuits, for example six, in a single I.C. chip thus making a very compact device useful in multiplex applications.
In practice a low on resistance switch FET is used in order to achieve low voltage offset errors. This means that the gate to substrate capacitance is correspondingly large. When switching, this capacitance must be charged to turn the switch off and discharged to turn the switch on. The driver circuitry therefore must be capable of handling large current transients while dissipating minimum power in the steady state conditions. The subject of U.S. Pat. No. 3,916,222 is in part a showing of how to discharge gate capacitance so as rapidly to turn the FET on using a low steady state power dissipation, particularly in the off switch state.
Another aspect of the prior art is the capability of integrating transistors and FETs in a common substrate. While all of the FETs fabricated simultaneously will tend to track each other in that they will all have similar pinch off voltage (Vp), the absolute value of this voltage can vary substantially in a mass production process. This makes matching FETs and transistors a difficult control process.
As pointed out above the actuation of a FET switch involves the charging and discharging of the gate capacitance and this can involve the transfer of substantial transient currents. This has the unfortunate effect of superimposing transients on the analog output line. As the circuit switching speeds are increased, these transients can become a real problem in the analog circuits.